Let's add a new attribute, wine_category, to both dataframes. Do you recall how we did that?

Of course, check the example code given as follows:

df_white['wine_category'] = 'white'
df_red['wine_category'] = 'red'

That was easy, right? Next, let's see what are the unique values of the column quality in both types of wines:

print('RED WINE: List of "quality"', sorted(df_red['quality'].unique()))
print('WHITE WINE: List of "quality"', sorted(df_white['quality'].unique()))

The output of the preceding code is given as follows:

RED WINE: List of "quality" [3, 4, 5, 6, 7, 8]
WHITE WINE: List of "quality" [3, 4, 5, 6, 7, 8, 9]

Note that both the red and the white wines have the same unique values for the quality column.